This invention relates to retroreflective articles comprising a curved low surface energy substrate, flexible retroreflective sheeting, and a layer of pressure-sensitive adhesive. More particularly, this invention relates to crosslinked tackified acrylic adhesives having good low temperature properties particularly suitable for adhesion to curved substrates having low surface energy.
Retroreflective materials have the property of redirecting incident light back towards its originating source. This advantageous property has led to the widespread use of retroreflective sheeting on a variety of articles. Very often retroreflective sheeting is used on flat inflexible articles, for example, road signs and barricades. However, situations frequently arise which require sheeting to be used on irregular or curved surfaces. For example, retroreflective sheeting may be adhered to irregular surfaces of traffic control devices, such as traffic cones, poles, barrels (drums), tubes, or nose cones or to vehicle bumpers or mud flaps, which requires the sheeting to adhere to a curved surface of varying circumference. In situations where the underlying surface is irregular or curved, the retroreflective sheeting desirably possesses good conformability and flexibility without sacrificing retroreflective performance.
There may also be situations where the underlying substrate expands and contracts at a different rate than the retroreflective sheeting. For example, for a temperature drop of 40xc2x0 C. (104xc2x0 F.), a traffic control device such as a low density polyethylene barrel would contract by about 0.80%, based on a coefficient of linear thermal expansion of 200xc3x9710xe2x88x926 (m/mK) at 20xc2x0 C. For the same temperature change, a retroreflective sheeting with a polycarbonate layer would contract by only about 0.23%, based on a coefficient of linear thermal expansion of 57xc3x9710xe2x88x926 (m/mK) at 20xc2x0 C. Thus, the barrel contracts almost 3.5 times more than the retroreflective sheeting. Because the retroreflective sheeting is wrapped outside of the barrel, conventional sheetings may wrinkle and lift off the barrel in response to the temperature change. In these situations, the retroreflective sheeting preferably accommodates the differences in thermal expansion and contraction without compromising retroreflectivity and without lifting off the substrate. Further, the adhesive between the sheeting and the barrel preferably accommodates these differences in thermal expansion and contraction.
Traffic control devices and vehicle parts often have low energy irregular or curved surfaces. In addition, these substrates are subject to weathering, temperature fluctuations, and impact from vehicles. Preferably, both the retroreflective sheeting and the adhesive used to secure the sheeting to these substrates perform well despite these constraints.
The two common types of retroreflective sheeting are microsphere-based sheeting and cube-corner sheeting.
Microsphere-based sheeting, sometimes referred to as xe2x80x9cbeadedxe2x80x9d sheeting, is well known in the art and employs a multitude of microspheres, typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. The microspheres are separate from each other and therefore do not severely hinder the sheeting""s ability to be flexed. Illustrative examples of such retroreflectors are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult).
Cube-corner sheeting typically uses a multitude of rigid interconnected cube-corner elements to retroreflect incident light. Many types of flexible cube-corner sheeting are known in the art. See for example U.S. Pat. No. 3,992,080 (Rowland), U.S. Pat. No. 4,576,850 (Martens), or U.S. Pat. No. 5,450,235 (Smith et al.).
A substantial number of pressure-sensitive adhesives (PSAs) are known in the art which have good adhesion to low energy surface substrates and include, but are not limited to, rubber-based adhesives, tackified KRATONS(trademark), non-polar acrylates, tackified acrylics, and polyalphaolefins.
However, not all of these adhesives perform satisfactorily on curved substrates (particularly those substrates which experience expansion and contraction or are subjected to high frequency impacts). This becomes an even greater problem at low temperatures, or upon exposure to chemicals and other roadway contaminants, or after exposure to other environmental weathering factors such as sunlight.
Delamination, buckling, and even pop-off of the flexible sheeting often occur because of adhesive failure. Thus, an article having an adhesive that performs suitably despite these constraints would be advantageous.
The present invention provides retroreflective articles comprising a curved substrate having a low energy surface, flexible retroreflective sheeting, and a layer of pressure-sensitive adhesive, and a method for making such articles. The article comprises:
(a) a curved substrate having a surface energy below about 38 dynes/cm2;
(b) flexible retroreflective sheeting; and
(c) a crosslinked tackified pressure-sensitive adhesive adhering said sheeting to said substrate, said adhesive comprising the reaction product of:
(i) from about 65 to about 99 parts by weight of at least one monomer selected from the group consisting of a first monofunctional acrylate or methacrylate ester of a non-tertiary alkyl alcohol, and mixtures thereof, the alkyl group of which comprises from about 4 to about 12 carbon atoms, which as a homopolymer has a glass transition temperature less than xe2x88x9225xc2x0 C.;
(ii) from about 1 to about 10 part(s) by weight of a polar monomer copolymerizable with the monomer(s) of component (i);
(iii) optionally from 0 to about 25 parts by weight of a non-polar ethylenically unsaturated monomer copolymerizable with components (i) and (ii) selected from the group consisting of alkyl (meth)acrylates, N-alkyl (meth)acrylamides, and combinations thereof, whose homopolymer has a solubility parameter of no greater than 10.50 and a Tg greater than 15xc2x0 C.;
(iv) from about 1 to about 65 parts by weight of a tackifier based upon the total weight of components (i), (ii), and (iii); and
(v) optionally from 0 to about 1 part by weight of a crosslinking agent based upon the total weight of components (i), (ii), and (iii)
wherein the adhesive contains sufficient amounts of components (ii) and (iv), and is sufficiently crosslinked, to:
(I) have a glass transition temperature less than about xe2x88x9215xc2x0 C. at a frequency of 1 rad/sec as measured by dynamic mechanical analysis;
(II) enable the sheeting, when wrapped around a low density polyethylene barrel, to remain firmly bonded to the barrel without buckling when cooled from 49xc2x0 C. to xe2x88x921xc2x0 C.; and
(III) provide an adhesive bond to such barrel that exhibits increased energy absorption when tested for impact resistance at 0xc2x0 C. according to ASTM D4272.
Curved substrates include, but are not limited to, a traffic post (having a radius of curvature of about 0.025 meters (1 inch)), a barrel (having a radius of curvature of about 0.23 meters (9 inches)), a nose cone (having a radius of curvature of about 0.91 meter (3 feet)) and a railroad car (having a radius of curvature of about 1.5 meters (5 feet)). Curvature is the magnitude of the rate of change of the direction of the curve with respect to arc length. The radius of curvature at a point is the reciprocal of the curvature at that point. (See Calculus and Analytical Geometry, Thomas, 4th ed., Addison Wesley.) The curved substrates of the present invention may have more than one radius of curvature. For example, the radius of curvature of a traffic cone varies with direction. Preferably, the curved substrates of the present invention have a radius of curvature ranging from about 2.5 cm (1 inch) to about 1.5 meters (5 feet).
The flexible sheeting may be either microsphere-based or cube-corner type and is preferably sufficiently flexible and conformable to be conformed to the surface of the substrate.
Another embodiment of the present invention is a method of making the articles of the present invention using pouched UV polymerized adhesive for extrusion coating.